Method of making laminate sheets for printed circuit boards

ABSTRACT

In preparing an epoxy resin laminate sheet by impregnating a nonwoven fabric of glass fiber with an epoxy resin to form a prepreg, laminating a predetermined number of so formed prepregs and pressing, while heating, the laminate, when the nonwoven fabric of glass fiber to be used as a substrate is treated with a liquid mixture containing a water-soluble phenol resin and a silane coupler prior to the impregnation step, a laminate sheet suitable for formation of a printed circuit board can be obtained at a low manufacturing cost. The so formed laminate sheet not only has excellent heat resistance, drilling workability and punching workability at room temperature but also is characterized by such an excellent property that when it is subjected to the boiling water treatment, the degree of reduction of electric properties such as the insulation resistance is very small and whitening of the substrate is not caused to occur.

FIELD OF THE INVENTION

This invention relates to a method for preparing epoxy-glass laminatesheets for printed circuit boards with use of a nonwoven fabric of glassfiber. More particularly, the invention relates to a method forpreparing epoxy-glass laminate sheets for printed circuit boards, whichcomprises impregnating a nonwoven fabric of glass fiber with an epoxyresin, drying the impregnated glass fibers to form an epoxyresin-impregnated glass fiber prepreg, laminating a predetermined numberof so prepared prepregs, and pressing, while heating, the resultinglaminate, wherein the glass fibers are, prior to being impregnated withepoxy resin, treated with a liquid mixture containing a silane couplerand a water-soluble phenol resin.

BACKGROUND OF THE INVENTION

Epoxy resin laminate sheets comprising as a substrate a nonwoven fabricof glass fibers have heretofore been prepared by a method comprising thesteps of impregnating a nonwoven fabric of glass fiber with an epoxyresin, drying the impregnated glass fibers to form a prepreg, laminatinga predetermined number of so formed prepregs, optionally piling a metalfoil such as copper foil on the laminate, and then pressing, whileheating, the laminate between mirror-polished iron plates. Laminatesheets prepared according to this conventional method with use of anonwoven fabric of glass fiber have good heat resistance comparable tothat of epoxy resin laminates prepared by employing a woven fabric ofglass fibers as a substrate, and they are excellent in drillingworkability and punching workability.

Epoxy resin laminate sheets prepared according to the above conventionalmethod by employing a nonwoven fabric of glass fiber as a substrate,however, are very poor in moisture resistance, and they are fatallydefective in that when they are subjected to the boiling water test,reduction of the insulation resistance is extreme and the substrate iswhitened. Therefore, when these laminate sheets are used for formationof printed circuit boards, they hardly satisfy requirement standardsspecified with respect to epoxy resin laminate sheets comprising asubstrate of a glass fiber woven fabric or a paper substrate for use inproduction of printed circuit boards.

It is believed that the above fatal defect of an epoxy resin laminatesheet comprising a nonwoven fabric of glass fiber as a substrate is dueto a binder used for formation of the nonwoven fabric of glass fiber.The nonwoven fabric of glass fiber used in this field includes a glassmat and a glass paper, and it is generally formed by shaping glassfibers having a diameter of several microns or shortly cut productsthereof into a thin sheet or paper. It is known that at this step, as amedium for bonding the glass fibers, there are employed (1) chemicals orresin (the product is hereinafter referred to as "substrate A") and (2)a cellulose fiber (the product is hereinafter referred to as "substrateB"). Such bonding medium of binder has bad influences on properties oflaminate sheets for use in producing printed circuit boards. A methodcomprising treating a nonwoven fabric of glass fiber with a silanecoupler may be considered as means for improving this defect. Accordingto this method, however, in the case of the substrate A a large amountof the silane coupler should be used for the treatment and themanufacturing cost becomes high, and in the case of the substrate B noimprovement of properties can be attained.

We have made investigations on epoxy resin laminate sheets comprising anonwoven fabric of glass fiber as a substrate, expecially those forprinted circuit boards, with a view to developing a method in which theforegoing fatal defect can be overcome and the properties of these epoxyresin laminate sheets can be improved at a lower cost, and as a resultit has now been found that good results can be obtained when a nonwovenfabric of glass fiber to be used to be used as a substrate is treatedwith a liquid mixture containing a watersoluble phenol resin and asilance coupler.

OBJECTS OF THE INVENTION

It is, therefore, a primary object of this invention to provide a methodof making an epoxy-glass laminate sheet for a printed circuit board inwhich such properties as the resistance to the boiling water test arehighly improved at a low cost though a nonwoven fabric of glass fiber isemployed as a substrate.

Another object of this invention is to provide a method of making anepoxy-glass laminate sheet comprising a substrate of a nonwoven fabricof glass fiber which is comparable or superior to conventionalepoxy-glass laminate sheets comprising a glass cloth substrate or papersubstrate with respect to the heat resistance, the drilling workability,the punching work-ability at room temperature and the resistance to theboiling water test.

Other objects, features and advantages of this invention will beapparent from the following description.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to an improvement of the method of makingepoxy-glass laminate sheets comprising a nonwoven fabric of glass fiberas a substrate, which comprises the steps of impregnating a nonwovenfabric of glass fiber with an epoxy resin, drying the resulting fabricof glass fiber to form a prepreg, laminating a predetermined number ofso formed prepregs, and then pressing, while heating, the laminate toobtain an epoxy-glass laminate sheet for production of a printed circuitboard, said improvement being characterized in that the nonwoven fabricof glass fiber is treated with a liquid mixture containing a silanecoupler and a water-soluble phenol resin prior to the step ofimpregnating the nonwoven fabric of glass fiber with the epoxy resin.

This invention will now be detailedly explained.

As described above, the method of this invention is characterized inthat a nonwoven fabric of glass fiber used as a substrate is treatedwith a liquid mixture containing a silane coupler and a water-solublephenol resin prior to impregnation of the nonwoven fabric with an epoxyresin. In general, the liquid mixture to be used for this treatment isformed by dissolving a water-soluble phenol resin and a silane couplerinto a mixed solvent of water and an alcohol. Water can also be employedas the solvent. The weight ratio of the water-soluble phenol resin tothe silane coupler in the liquid mixture may be, in a solid base,1/0.01-0.5 for use for the substrate A and 1/0.001-0.1 for use for thesubstrate B. The concentration of the liquid mixture may be 1 - 10 % byweight. When too large an amount of a silane coupler is contained in theliquid mixture, the properties of the obtained substrate are improvedbut, disadvantageously the cost becomes comparatively high and the lifeof the liquid mixture is shortened. Then use of an alcohol as acomponent of the solvent for the liquid mixture makes it easy to form aprepreg after impregnation of the substrate with epoxy resin. The finalproduct laminate sheet obtained through the use of alcohol is notdifferent in properties from that obtained through the use of wateronly. Examples of alcohols to be employed as a component of the solventinclude methyl alcohol, ethyl alcohol and isopropyl alcohol. When themixed solvent of water and an alcohol is used, the mixing weight ratiois not critical but water/alcohol = 1/0.5 - 2.0 is preferably employed.As the silane coupler, there can be mentioned, for example,γ-ethylenediaminopropyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltrimethoxysilane andγ-chloropropyltrimethoxysilane. Any of water-soluble phenol resins canbe used in this invention. For example, there is preferably employed aphenol resin prepared by reacting a phenol with formaldehyde orparaformaldehyde in the presence of a catalyst at 40° - 100°C. for 2 -50 hours and composed mainly of dimers and trimers. As the phenol, theremay be employed phenol, cresol, xylenol, isopropylphenol,tert.-butylphenol, octylphenol, bisphenol A and the like. Examples ofthe catalyst include an alkali such as sodium hydroxide or potassiumhydroxide, ammonia, or an amine such as triethanolamine.

A nonwoven fabric of glass fiber is immersed in the so formed treatingliquid and then the treating liquid-impregnated nonwoven glass fabric isdried at 120° - 150°C. for 5 - 15 minutes. Then, the so treated nonwovenglass fabric is impregnated with an epoxy resin and then dried to obtaina prepreg of the so called B-stage. As the epoxy resin, there may beemployed those as employed in the conventional art (See, for exampleJapanese patent publication No. 3237/1973). With use of a suitable orpredetermined number of the so prepared prepregs, a laminate sheet isprepared according to the customary method known in the art (See, forexample Japanese patent publication No. 47109/1972). The so preparedlaminate sheet is characteristic over a conventional laminate sheetcomprising as a substrate a nonwoven fabric of glass fibers in that whenit is immersed in boiling water, reduction of the electric resistance isvery small and no whitening is observed in the substrate.

As described, in conventional epoxy-glass laminate sheets comprising asa substrate a nonwoven fabric of glass fiber, there are brought aboutsome disadvantages when they are immersed in boiling water. For example,in the case of the substrate A, a chemical or resin used as a binder isreadily dissolved out in boiling water, and moisture is readily absorbedin voids formed by dissolving-out of the binder or glass fibers arereadily peeled off from the epoxy resin, with the result that electricproperties are drastically degraded and the substrate is whitened. Inorder to prevent occurrence of such undesired phenomena, in thisinvention it is indispensable that a nonwoven fabric of glass fiber tobe used as a substrate is treated with the above-mentioned treatingliquid containing a water-soluble phenol resin and a silane coupler. Inthe case of the substrate A, it is believed that when the substrate istreated with the treating agent of this invention, the bonding mediumsuch as a chemical or resin is coated with the treating agent orinsolubilized thereby, and the bonding between the glass fibers andepoxy resin is reinforced by the silane coupler. As a result of thisdissolving-out of the binder and peeling of the glass fiber from theepoxy resin can be effectively prevented at the boiling water treatment.Further, in the case of the substrate B, the moisture-absorbing propertyof cellulose fibers incorporated as the bonding medium is effectivelylowered by the treatment of this invention and the bonding between theglass fibers and epoxy resin is effectively improved. Thus, theabove-mentioned defects involved in conventional epoxy resin laminatesheets comprising as a substrate a nonwoven fabric of glass fibers canbe completely overcome according to the method of this invention, andepoxy resin laminate sheets suitable for production of printed circuitboards can be conveniently prepared by employing a nonwoven fabric ofglass fiber as a substrate according to the method of this invention.

The above description is made in respect of the epoxy-glass laminatesheet comprising a prepreg produced using a nonwoven fabric of glassfiber which was pretreated according to this invention and thenimpregnated with an epoxy resin. However, it is to be noted that theprepreg produced using a woven fabric of glass fiber which wasoptionally pretreated and impregnated with an epoxy resin may beemployed together with the prepreg produced according to this inventionto produce an epoxy-glass laminate sheet; for example, the prepregproduced using the woven fabric may be attached onto the surface ofpresent prepreg produced using nonwoven fabric or may be sandwichedbetween the present prepregs to produce another type of epoxy-glasslaminate sheet.

This invention will now be illustrated more detailedly by reference toExamples, which by no means limit the scope of this invention.

EXAMPLE 1

Glasstex 936 G (nonwoven fabric of glass fiber manufactured and sold byMead Co., U.S.A.) was impregnated with an about 1 % aqueous solution ofa mixture of γ-ethylenediaminopropyltrimethoxysilane/water-solublephenol resin (solid component weight ratio = 0.01-0.1/1) and theimpregnated nonwoven fabric of glass fiber was dried in air at 140° -150°C. for 10 - 15 minutes. As the water-soluble phenol resin, the resinwhich had been obtained by reacting 1 mol of phenol and 1.4 mol offormaldehyde in the presence of 0.1 mol of trimethylamine at 85°C. for 6hours was employed (in Examples 2 and 3 also, the same was employed).The so treated Glasstex 936 G was further impregnated with an epoxyresin solution (formed by dissolving 100 parts by weight of an epoxyresin having 400 - 500 epoxy equivalents, 4 parts by weight ofdicyandiamide and 0.2 part by weight of benzyldimethylamine into 60parts by weight of a mixed solvent of methyl glycol and methyl ethylketone) and was dried at about 130° to about 150°C. for 10 - 15 minutesto obtain a prepreg of the B-stage. As the epoxy resin, EPIKOTE 1001(trade name of epoxy resin manufactured by Shell Chem. Corp.) isemployed. Then, about 20 sheets of so formed prepregs were piled and hotpressed at a temperature of about 80°C. under a pressure of about 20Kg/cm² in the state inserted between mirror-polished iron plates toobtain a laminate sheet having a thickness of about 1.6 mm.

In order to examine electric properties of the laminate sheet and thedegree of whitening of the substrate, the so obtained laminate sheet wasdipped in boiling water for 120 minutes according to the test method ofJIS C-6481, and the insulation resistance was measured and theappearance was inspected. It was found that the insulation resistancewas (1.0 - 5.0) × 10¹⁵ Ω under normal conditions and it was reduced to(1.0 - 5.0) × 10¹⁰ Ω by the boiling water treatment. The whitening wasnot observed on the substrate even after the boiling water treatment.

For comparison, a laminate sheet was prepared from Glasstex 936 G in thesame manner as described above except that the treatment of thisinvention was not conducted. The insulation resistance of the sheet was(1.0 - 5.0) × 10¹⁵ Ω under normal conditions and it was reduced to(1.0 - 5.0) × 10⁷ Ω by the above boiling water treatment. Further, thesubstrate was whitened in this comparative laminate sheet by the aboveboiling water treatment.

For another comparison, a laminate sheet was prepared from Glasstex 936G in the same manner as described above except that the Glasstex wastreated with an aqueous solution containingγ-ethylenediaminopropylmethoxysilane alone. In this case, it was foundthat in order to obtain a product having properties equivalent to thoseof the above laminate sheet prepared by treating Glasstex 936 Gaccording to this invention, it was necessary to increase the silaneconcentration in the treating liquor to above 2.0 %.

EXAMPLE 2

Glasstex 936 G was impregnated with an about 2 % aqueous solution of amixture of γ-glycidoxypropylmethoxysilane/water-soluble phenol resin(solid component weight ratio = 0.1 - 0.5/1) and the impregnatednonwoven fabric of glass fiber was dried in air at 140° - 150°C. for10 - 15 minutes. From the so treated fabric of glass fiber, a laminatesheet was prepared under the same conditions as in Example 1 and wassubjected to the same test as described in Example 1. The insulationresistance of the laminate sheet was (1.0 - 5.0) × 10¹⁵ Ω under normalconditions and it was reduced to (3.0 - 7.0) × 10⁹ Ω by the boilingwater treatment. It was found that no substantial whitening was causedto occur in the substrate by the boiling water treatment.

EXAMPLE 3

A nonwoven fabric of glass fiber of the substrate B type (containing 40% of cellulose fibers) was immersed in an about 10 % solution of amixture of γ-ethylenediaminopropyltrimethoxysilane/water-soluble phenolresin (solid component weight ratio = 0.01 - 0.1/1) in the mixed solventof water and methyl alcohol (1/1 by weight) and was then dried at 140° -150°C. for 10 - 15 minutes. The so treated nonwoven fabric of glassfiber was impregnated with the same epoxy resin solution as used inExample 1 and dried at 130° - 150°C. for 10 - 15 minutes to obtain aprepreg of the B-stage. Then, about 15 sheets of so prepared prepregswere piled and hot pressed under the same conditions as adopted inExample 1 to obtain a laminate sheet having a thickness of about 1.6 mm.It was found that the insulation resistance of the laminate sheet was(1.0 - 5.0) × 10¹⁵ Ω under normal conditions and it was reduced to(1.0 - 3.0) × 10¹¹ Ω by the boiling water treatment described inExample 1. Whitening of the substrate was not observed at this boilingwater treatment.

For comparison, a laminate sheet was prepared in the same manner asabove except that the treatment of this invention was not conducted. Theinsulation resistance of the comparative laminate sheet was (1.0 - 5.0)× 10¹⁵ Ω under normal conditions and it was reduced to (5.0 - 9.0) × 10⁶Ω by the boiling water treatment. Further, the substrate of thecomparative laminate sheet was whitened by the boiling water treatment.

When examples 1 and 2 were repeated by employing Hiaron GP (manufacturedand sold by Tokushu Seishi K.K.) and Glass Paper GMN (manufactured andsold by Honshu Seishi K.K.) instead of Glasstex 936 G, similar resultswere obtained.

As is apparent from the results of the foregoing Examples, anepoxy-glass laminate sheet prepared from a nonwoven fabric of glassfiber treated with both a silane coupler and a water-soluble phenolresin according to this invention is characteristic over an epoxy-glasslaminate sheet prepared from an untreated nonwoven fabric of glass fiberin that the degree of reduction of the insulation resistance by theboiling water treatment is much smaller and the insulation resistancecan be retained at a much higher level after the boiling watertreatment, and that in this laminate sheet prepared according to thisinvention, no substantial whitening of the substrate is caused to occurby the boiling water treatment. Further, this epoxy-glass laminate sheetprepared according to this invention can also fully meet the generalstandard requirements specified with respect to epoxy-glass laminatesheets comprising glass fiber cloth substrate, a paper substrate or thelike.

The method of this invention has such advantages over the conventionalmethod comprising treating a nonwoven fabric of glass fiber with asilane coupler alone that the amount of the treating agent necessary forattaining the desired effect is much smaller and the treatment cost canbe much reduced.

What is claimed is:
 1. In a method of making an epoxy-glass laminatesheet comprising a nonwoven fabric of glass fiber as a substrate, whichcomprises the steps of impregnating a nonwoven fabric of glass fiberwith an epoxy resin, drying the resulting fabric of glass fiber to forma prepreg, laminating a predetermined number of so formed prepregs, andthen pressing, while heating, the laminate to obtain an epoxy-glasslaminate sheet for production of a printed circuit board, theimprovement wherein the nonwoven fabric of glass fibers is treated witha liquid mixture containing a silane coupler and a water-soluble phenolresin prior to the step of impregnating the nonwoven fabric with theepoxy resin.
 2. A method according to claim 1 wherein said liquidmixture contains as a solvent water or a mixture of water and analcohol.
 3. A method according to claim 1 wherein said silane coupler isa member selected from the group consisting ofγ-ethylenediaminopropyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltrimethoxysilane andγ-chloropropyltrimethoxysilane.
 4. A method according to claim 1 whereinsaid phenol resin is one prepared by reacting phenol with formaldehydein the presence of a catalyst at 40° - 100°C. for 2 - 50 hours andcomposed mainly of dimers and trimers.
 5. A method according to claim 1wherein said nonwoven fabric of glass fiber is one in which a chemicalor resin is used as a medium for bonding the glass fiber.
 6. A methodaccording to claim 1 wherein said nonwoven fabric of glass fiber is onein which cellulose fibers are incorporated as a medium for bonding theglass fibers.